Hermetically sealed harmonic drive transmission manufacture

ABSTRACT

The present invention comprises a harmonic drive transmission assembly for use in contamination free manufacturing fields such as computer chip making. The harmonic drive unit includes a flexspline having a diaphragm at one end and an outwardly flared lip adjacent its other end which is integrally connected to a mounting flange. The flexspline including the diaphragm and mounting flange are all integral with one another, all being machined from a single piece of steel bar stock.

This is a divisional of application Ser. No. 08/533,394, filed on Sep.25, 1995 now U.S. Pat. No. 5,642,645.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to harmonic drive transmissions, and moreparticularly to the manufacture and use of hermetically sealed harmonicdrive transmissions.

(2) Prior Art

Harmonic drive devices have been utilized for quite a few years,wherever a lightweight, compact, high ratio transmission of power isneeded. All harmonic drive transmissions employ three basic elements.Those elements are a circular spline, a flexspline, and a wavegenerator, assembled normally with the circular spline fixed in anon-rotating arrangement, with the flexspline rotating as the outputelement. The wave generator, which typically functions as the inputelement, would be rotated by a primary power source. Teeth on theflexspline and the circular spline are cut to the same circular pitch.However, the flexspline is somewhat smaller in circumference than thecircular spline, and it has fewer teeth. The numerical difference in thenumber of teeth is always a multiple of the number of lobes on the wavegenerator.

An early example of a harmonic drive transmission, is shown in U.S. Pat.No. 2,929,26 to C. W. Musser, was issued Mar. 22, 1960, the Patent beingentitled "StrainWave Gearing Tubular Shaft". Since that time, harmonicdrive transmissions have been used for azimuth drives or antennasystems, or rotary cable drives for welding positioners, for valveactuators, for consumer products such as door openers or actuators orautomotive closures, for microscope focusing knobs or for steeringmechanisms on rocket booster propulsion systems.

There are certain industries, however, where rotary power is needed, yetcontamination from any part of the machinery within the worked fieldwould be critical to the work product. The provision of positive,accurately controlled transmission of motion through an uninterruptedmetallic barrier is desired, also in those certain fields. Since nopacking, bellows or fallible seal is used, some containment ofcontamination utilizing a harmonic drive transmission, may be obtained.Such a unit is shown in U.S. Pat. No. 3,196,713 to H. A. Robinson,issued in Jul. 27, 1965, entitled "Hermetically Sealed Transmissions".Such a unit may provide typical rotary-to-rotary power utilizing thesame three basic components of harmonic drive transmissions, but theirconfiguration has been altered to meet the specific feed throughrequirement. The flexspline is a flexible walled tube with one endclosed by welding its edge to a disc or diaphragm, and the other end ofthe tube is bolted or welded to a mounting flange. The wave generator,operating inside the stationary flexspline deflects the walls of thetube and flexspline combination into engagement with the circularspline, which functions as the output member and delivers a rotation inthe same direction as the input.

It is an object however of the present invention, to provide animprovement over the hermetically sealed transmissions of the prior artharmonic drive units.

It is yet a further object of the present invention, to provide ahermetically sealed harmonic drive transmission which may be utilized ina field where contamination of any sort would be absolutely critical tothe product being produced.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a process of the manufacturing of ahermetically sealed harmonic drive transmission which will be utilizedin environments where contamination would be critical, such as in theelectronic industry in the manufacture of computer chips.

In the manufacturing of the present invention, a flexspline of a flaringshape will be shown, of a shape similar to that shown in U.S. Pat. No.3,196,713, incorporated here in the reference, though it would beunderstood that a flexspline of different shape may be employed, forexample, one cup-shaped.

In accordance with the features of the present invention, the closed endflexspline and the mounting flange to which a normal flexspline would beotherwise attached, is made of one unitary piece of forging. Such apiece forging is made from a bar of stainless steel shaped somethinglike a bell. The forging is placed in a turning center of an appropriatemilling machine, on which the outside diameter is machined down to arough tooling dimension. The bar is then re-chucked on that outsidediameter. Then the inside diameter may be drilled to its rough internaldiameter. The solid bar comprising the flexspline is again re-chucked,located against its dimension to another tooling dimension on its roughexternal shape.

The rough flexspline stock is next re-chucked on that tooling dimension,to permit the final finish internal configuration to be machined. Theflexspline now has an end cap, called a diaphragm, which is solid andintegral with the walls of the bell shaped flexspline, and the walls ofthe flexspline critically are also integral with its mounting flange.

The outside final diameter of the combination flexspline with theintegral mounting flange and diaphragm is now finished by placing theflexspline assembly on a turning mandrel. The combination mandrel andflexspline assembly is placed into an alignment fixture. One end of thealignment fixture includes a housing having a piston with a springloaded cup which presses against the diaphragm of the flexsplineassembly. A vacuum is created in the head of the alignment fixture toevacuate air from the bore of the flexspline assembly.

The alignment mandrel, as known in the art, is utilized to support thehollow flexspline cup during the machining operation in which the outerwall is turned down to 0.010 inches and the diaphragm is turned down to0.020 inches. A low melting point alloy steel known as Sera Metal 117,sold by the Sera Metal Company, is poured into the bottom of the holdingmandrel. The low melting point of alloy steel runs through a pluralityof channels in the holding mandrel, and into the bore of the flexsplinecup.

The holding mandrel with the flexspline cup assembly thereon, having thelow melting point alloy steel therewithin, is installed onto a lathe forfinal turning of the walls to the thin outer diameter.

After the diaphragm, the flexspline walls, and mounting flange are allturned to the appropriate outer configuration, the mandrel with theflexspline assembly thereon is removed and the combination is heated tojust above the melting pointing of the low melting point alloy steel, tofor example, about 130 degrees Fahrenheit, wherein the mandrel and alloyare removed from the flexspline assembly.

The harmonic drive unit includes a first drive shaft having a distal endwhich is secured to a flange, which flange is bolted to the first closedend of a circular spline. The circular spline comprises an annular ringhaving a second end on which there is disposed an array of circularspline teeth. The closed end of the flared or bell shaped flexspline isdisposed within the bore of the circular spline. The flexspline includesa cup shaped wall having a diaphragm across its distal end. The bellshaped or flared end of the flexspline is integral with the mountingflange. The flexspline assembly thus critically being made of oneunitary piece of stock to absolutely prevent leakage of gases orcontaminants through the flexspline, which could occur if the diaphragmwere welded to the wall of the flexspline. A solid forging of stock alsopermits the grains of the steel forming the walls and diaphragm to beoriented in a more leakproof way (that is, parallel to the walls) thanif the flexspline were made any other way. The annular array of flex-spline teeth is engaged with the circular spline teeth. The flexsplineteeth are disposed on the outwardly radially directed edge of the wallsof the flexspline. An annular wave generator is disposed radiallyinwardly of the flexspline teeth. A wave generator is mounted on aninput shaft which extends through a wall opening in a housing, whichhousing is utilized for the manufacture of contamination-freecomponents, such as computer chips or sophisticated electronic devices.The annular wave generator has two or more lobes thereon, as is known inthe harmonic drive transmission field. A ball bearing assembly isdisposed on the radially outer periphery of the annular wave generator.The bearing assembly comprises an inner bearing race, an outer bearingrace and plurality of roller balls spaced circumferentiallytherebetween.

The mounting flange which is critically integral to the flexspline, maybe welded or bolted along its annular periphery, to the wall of thehousing in which the products to be made, to insure contamination freeutilization of the harmonic drive transmission capabilities. The weldingalong this juncture does not have flexing forces acting upon it as doesthe diaphragm and flexspline walls.

In operation of the harmonic drive assembly, as is known in the art, thedrive shaft rotates the circular spline about the stationary bell shaped(flared) flexspline. Rotation about the flared flexspline induces a wavemotion within the flexspline teeth and hence a rotary motion within theannular wave generator. That rotary motion within the annular wavegenerator causes rotary motion within the input shaft, to drive theappropriate mechanism within the housing of the contamination freeproduction area.

The invention thus comprises a method of manufacturing of a harmonicdrive flex-spline assembly, including a flexspline cup having anintegral diaphragm at one closed end and an integral mounting flange atits other open end, comprising the steps of arranging a solid steelforging in a chuck arrangement in a metal turning machine, drilling acenter bore within the solid steel forging, turning the external surfaceof the solid steel forging to a preset outside dimension, machining theintegral mounting flange on the open proximal end of the flexsplineassembly, to its finished outer configuration. The method also includesthe steps of placing the rough turned solid steel forging into a machinetool for full bore drilling of the internal diameter of said flexsplinecup assembly, placing the finished internal bore onto a turning mandrelfor support of said flexspline assembly, suction drawing a low meltingpoint alloy steel into the flex-spline cup assembly for internal annularsupport of the flexspline walls during subsequent machining operations,supporting the turning mandrel in a turning machine, while turning theflexspline walls down to its finished dimension of about 0.010 inchesand the integral diaphragm down to a thickness of 0.020 inches.

In another aspect of invention comprises a harmonic drive transmissiondevice for the transmission of rotary energy from an input source to anoutput mechanism within a wall of a sealed housing while providing ahermetically sealed relationship to the output shaft within the sealedhousing, comprising an input shaft having a wave generator arranged atone end thereof, the wave generator rotatively disposed within a unitaryflexspline having a unitary diaphragm and mounting flange therewith, acircular spline rotatively supported around the flexspline in a geartooth motion communicating relationship therewith, the flexspline havinga side wall, the diaphragm and the mounting flange all machined from asingle solid forging of steel, so that the arrangement of gear teeth onthe flexspline and the circular spline engage one another in anadvancing wave, to permit rotary motion to be provided to an outputshaft, which output shaft actuates a mechanism within a hermeticallysealed contamination-free production housing. The unitary mountingflange is attached to the inner side of said housing, to minimize thelikelihood of contamination from components within the flexsplineitself.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the present invention will become moreapparent, when viewed in conjunction with the following drawings, inwhich:

FIG. 1 is a side elevational view of a flexspline forging with the outerdiameter profile steps overlaid thereon showing such outer profiles theflexspline is constructed according to the principles of the presentinvention;

FIGS. 2 a, b, c, and d, are views similar to FIG. 1, showing the stepwise process in the manufacture of a flexspline assembly according tothe principles of the present invention; and

FIG. 3 is a side elevational view, partly in section, of part of aharmonic drive assembly secured to a housing where contamination freeparts are manufactured.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a process of the manufacturing of ahermetically sealed harmonic drive transmission which will be utilizedin environments where contamination would be critical, such as in theelectronic industry in the manufacture of computer chips.

In the manufacturing of the present invention, a flexspline of a flaringor bell shape will be shown, of a shape similar to that shown in U.S.Pat. No. 3,196,713, incorporated here in the reference, though it wouldbe understood that a flexspline of different shape may be employed, forexample, one cup-shaped.

In accordance with the features of the present invention, the flexsplineand the mounting flange to which a normal flexspline would be attached,is made of one unitary piece of forging 10, as shown in FIG. 1. Such apiece of forging 10 is made from a bar stock of stainless steel. Such abar is placed in a chuck of an appropriate turning center of a lathe.The flexspline 15 is then machined to its rough external shape and laterto is final external shape, as shown by the outside profile lines ofstep 1 "a" and step 2 "b". The rough outline is also shown in FIG. 2a.The initial rough internal bore diameter is shown as dashed line 14 inFIG. 2a, and the next rough bore shape is shown by line 16 in FIG. 2b.

The rough flexspline stock is next re-chucked to permit the final insidefinish internal bore configuration to be machined to the shape indicatedby the dashed lines 18, as shown in FIG. 2c. The flexspline 15 now hasan "end cap" 20, which includes the diaphragm 17, which is solid andintegral with the walls of the flute (bell) shaped flexspline 15. Thewalls 19 of the flexspline critically are also integral with itsmounting flange 22, as shown in FIG. 2d.

The outside final diameter of the combination flexspline 15 with theintegral mounting flange 22 and diaphragm 17 is finished by placing theflexspline assembly 15 on a turning mandrel, not shown. The combinationmandrel and the flexspline assembly 15, are placed into an alignmentfixture, also. One end of the alignment fixture includes a housinghaving a piston with a spring loaded cup which presses against thediaphragm of the flexspline assembly. A vacuum is created in the head ofthe alignment fixture to evacuate air from the bore 14 of the flexspline15 to permit the Cerra metal to totally fill the bore of the flexspline15.

The alignment mandrel, as known in the art, is utilized to internallysupport the hollow flexspline 15 during the machining operation. In thatoperation, the outer wall 19 is turned down to about 0.010 inches andthe diaphragm 17 is turned down to about 0.020 inches, as shown infinished state in FIG. 2d. A low melting point alloy steel known asCerro 117 steel, is ladelled into the bottom of the holding mandrel. Thelow melting point of alloy steel runs through a plurality of channels inthe alignment mandrel, and into the bore of the flexspline 15, to fillup the evacuated space and internally support the flexspline 15.

The holding mandrel with the flexspline assembly 15 thereon, having thelow melting point alloy steel therewithin, is installed onto a lathe,for final turning of the walls 19 and diaphragm 17, to the thin outerdimensions.

After the diaphragm 17, the flexspline walls 19, and mounting flange 22are all turned to the appropriate outer configuration, as shown in FIG.2d, the mandrel with the flexspline assembly 15 thereon is removed andthe combination is heated to just above the melting pointing of the lowmelting point alloy steel, for example, to about 130 degrees Fahrenheit,wherein the mandrel and alloy are removed from the flexspline assembly15.

The finished flexspline 15 is a component of a harmonic drive assembly30, as shown in FIG. 3, which includes a first output shaft 32 having adistal end 34 which is secured to a flange 36. The flange 36 is boltedto the first closed end 38 of a circular spline 40. The circular spline40 comprises a rigid annular housing having, a second end 42 on whichthere is disposed an array of radially inwardly directed circular splineteeth 44. The closed diaphragm end 17 of the flared flexspline 15 isdisposed within the bore of the circular spline 40. The flaredflexspline 15 includes a cup shaped wall having its diaphragm 17 acrossits distal end. The flared end 21 of the flexspline 15 critically isintegral with the mounting flange 22. The flexspline assembly thuscritically being made of one unitary piece of stock to absolutelyprevent leakage of gases or contaminants through the flexspline 15,which could occur if the diaphragm 17 and the walls 19 were separatepieces welded to one another to make the flexspline, as is done in theprior art. A machined unitary flexspline and mounting flange machinedfrom a solid forged bar also permits the grains of the steel forming thewalls and diaphragm to be oriented (parallel to the walls) in a moreleakproof way than if the flexspline were made any other way. Theannular array of flexspline teeth 46 is engaged with the circular splineteeth 44. The flexspline teeth 46 are disposed on the outwardly radiallydirected edge of the walls 19 of the flexspline 15.

An annular wave generator 50 is disposed radially inwardly of theflexspline teeth 46, as shown in FIG. 3. The wave generator 50 ismounted on an input shaft 54 which extends through a wall opening 52 ina housing 56, which housing 56 may be utilized for the location andcontainment/manufacture of contamination-free components, such ascomputer chips or sophisticated electronic devices. The annular wavegenerator 50 has two or more lobes thereon, as is known in the harmonicdrive transmission field. A ball bearing assembly 60 is disposed on theradially outer periphery of the annular wave generator 50. The bearingassembly 60 comprises an inner bearing race 62, an outer bearing race 64and plurality of roller balls 66 spaced circumferentially therebetween.

The mounting flange 22 which is critically integral to the flexspline15, may be welded or bolted along its annular periphery, to the wall ofthe housing in which the products to be made, to insure contaminationfree utilization of the harmonic drive transmission capabilities.

In operation of the harmonic drive assembly 30, as is known in the art,the input shaft 54 rotates the lobed wave generator within thestationary flared flexspline 15. The mounting flange sealing the openend of the flexspline to a housing. The mounting flange beingconsiderably heavier and thicker than the flexspline, sufficient toprovide the anchoring and stability needed, while the walls of theflexspline where the radially outwardly directed teeth are, may have awave-like motion imparted therein. Rotation of the wave generator 50within the flared flexspline 15 thus induces rotary motion within thecircular spline 40 and hence a rotary motion within the output shaft 32.That rotary motion within the output shaft 32 causes rotary motion todrive an appropriate gearing 37 to turn production mechanisms such as acarousel 39 or X-ray lithography equipment for the manufacture ofelectronic chips or like substrates where contamination from anythingincluding that which might come from gear transmissions and get withinthe housing 56 of the contamination free production area. The harmonicdrive assembly 30 in the prefferred embodiment is arranged on the innerside of the wall comprising the housing 56, so that the bearings 66, thewave generator 50 and the input shaft 54 are not being exposed to anyatmosphere within the housing 56, which might otherwise contaminate anyproduct being manufactured therewithin, and the evacuated area of theproduction housing is exposed only to the area radially outwardly of theflexspline, and not to the bearings and associated mechanismstherewithin.

We claim:
 1. A method of manufacturing of a harmonic drive flexsplineassembly, including a flexspline cup having an integral diaphragm at oneclosed end and an integral mounting flange at its other open proximalend, comprising the steps of:arranging a solid steel forging in a chuckarrangement in a metal turning machine; drilling a center bore withinsaid solid steel forging; turning the external surface of said solidsteel forging to a preset outside flexspline wall dimension; machiningsaid integral mounting flange on said open proximal end of said flexspline assembly, to its finished outer configuration.
 2. The method ofclaim 1, also including the steps of:placing said forging into a machinetool for full bore drilling of the internal diameter of said flexsplinecup; placing the full bore onto a turning mandrel for support of saidflexspline assembly; suction drawing a low melting point alloy steelinto said flexspline cup, for internal support of said flexspline duringsubsequent machining operations; supporting said turning mandrel in aturning machine, while turning the flexspline wall down to a finisheddimension thickness of about 0.010 inches and said integral diaphragmdown to a thickness of 0.020 inches.